This invention relates to high temperature electrochemical cells that use molten alkali metals as negative electrode material and molten chalcogens as positive electrode materials. An ionically conductive glass electrolyte composition separates the electrodes and permits the electrochemical reaction to occur. In particular, this invention relates to the use of sodium-sulfur electrochemical cells with glass electrolytes of soda, zirconia, alumina and silica.
Sodium-sulfur electrochemical cells are most often provided with an electrolyte of crystalline beta alumina which is characterized by high ionic conductivity. However, this material is frangible and requires difficult and expensive fabrication techniques. Furthermore, it has not endured well particularly at grain boundaries on long exposure to molten sulfur, sodium and polysulfides at high temperature conditions.
Amorphous glass electrolytes have good fabrication properties but exhibit low ionic conductivities and often are unstable in sodium-sulfur cell environments. In one program sponsored by U.S. Department of Energy under Contract No. DE-AC02-76ET25103, a hollow glass fiber was developed as electrolyte for a sodium-sulfur secondary electrochemical cell. However, due to its very low ionic cnductivity, e.g. at 300.degree. C., 4-5.times.10.sup.-5 (ohm-cm).sup.-1, very small diameter, elongated fibers were required to provide a large electrolyte surface area and a short ion migration path.
In a recent effort by Kucera et al., U.S. Pat. No. 4,544,614, Oct. 1, 1985, an ionically conductive glass with conductivies of more that 2.times.10.sup.-3 ohm-cm.sup.-1 at 300.degree. C. were prepared. Though the use of a glass with this greatly improved ionic conductivity, the inventors have discovered substantial improvements that can be incorporated into an electrochemical cell design.